An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems.
Capacitive-sensing MEMS accelerometer designs are highly desirable for operation in high gravity environments and in miniaturized devices, and due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a two layer capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure two distinct capacitances to determine differential or relative capacitance.
FIG. 1 shows a side view of a prior art asymmetric capacitive accelerometer 20 constructed as a conventional hinged or “teeter-totter” type sensor. Capacitive accelerometer 20 includes a static substrate 22 having metal electrode elements 24 and 26 of a predetermined configuration deposited on the surface to form respective capacitor electrodes or “plates.” A movable element 28, commonly referred to as a “proof mass,” is flexibly suspended above substrate 22 by a torsional suspension element 30 and rotates about a rotational axis, represented by a bi-directional arrow 32. A section 34 of movable element 28 on one side of rotational axis 32 is formed with relatively greater mass than a section 36 of movable element 28 on the other side of rotational axis 32. The greater mass of section 34 is typically created by offsetting rotational axis 32 from a geometric center 38 of movable element 28. Due to the differing masses on either side of rotational axis 32, movable element 28 pivots or rotates in response to acceleration, thus changing its position relative to the static sense electrodes 24 and 26. This change in position results in a change in electrical capacitance between movable element 28 and each of electrodes 24 and 26. Capacitors 40 and 42 shown in FIG. 1 represent this capacitance, or more particularly the change in capacitance, as movable element 28 pivots in response to acceleration. The difference between the capacitance, i.e., a differential capacitance, is indicative of acceleration. It should be understood that capacitors 40 and 42 are symbolic of this capacitance, and are not physical components of accelerometer 20.
When accelerometer 20 is subjected to high acceleration, movable element 28 can rotate and contact substrate 22, and thus be prevented from further rotation. This stopping feature is useful for avoiding structural failure of movable element 28 and/or to avoid shorting in either of capacitors 40 and 42. In some embodiments, a stop or post structure 44 may be implemented on opposing longitudinal ends 46 of movable element 28 as the stop feature. The relatively small surface area of stops 44 largely prevents movable element 28 from becoming stuck to the underlying electrodes and/or substrate. That is, without stops 44, the larger surface area of movable element 28 may become stuck to the underlying electrodes and/or substrate thereby rendering accelerometer 20 unusable.
FIG. 2 shows a chart of an exemplary output signal 46 of the asymmetric accelerometer 20 (FIG. 1) under sinusoidal excitation. Due to its asymmetrical configuration and the asymmetrical placement of stops 44, movable element 28 stops at a different acceleration amplitude between the positive and negative direction. The stop results in clipping at a positive acceleration value 48 that differs from the clipping at a negative acceleration value 50. This creates a non-zero time-averaged output value 52 in the overload response. A time-averaged output value of a sinusoidal excitation should be zero, so non-zero value 52 can cause inaccuracies in acceleration measurements, and possible malfunction of the device into which accelerometer 20 is incorporated.
Referring back to FIG. 1, sometimes an actuation electrode (not shown) is disposed on substrate 22 below section 34 of movable element 28 and beside electrode 24. Such an actuation electrode can be used for self-test activities. Unfortunately, the use of a single actuation electrode on one side of rotational axis 32 only allows for self-test in a single direction. Some methodologies call for actuation in both directions, i.e., a bi-directional self-test. With the asymmetric configuration of accelerometer 20, one technique for enabling bi-directional self-test is to reduce the effective areas of both of electrodes 24 and 26 and utilize some of that area for an additional pair of actuation electrodes. Unfortunately, the loss of area of electrodes 24 and 26 can result in accelerometer performance degradation.